D9 RGBM implementation

[IanH]

A combination of a new D9, taking my PADI Divemaster decompression theory exam and an enquiring mind has got me interested in decompression theory.

The consensus seems to be that Suunto computers are generally on the conservative side and are (or were) not full RGBM implementations.

2 questions really :-

Does the D9 address this issue i.e. it has deepstops so does this imply it a full implementation ?

More importantly if you turn on the deepstops and stick to them does the D9 become less conservative. What I'm driving at here is that it seems to me a partial implementation of RGBM (particularly one without deepstops) does not play to the strengths of the RGBM model so its not surprising those computers that partially implement RGBM get a reputation for conservatism. Whereas a full implementation (with deepstops) plays to the strengths of the model i.e. will following an RGBM profile with deepstops give you longer bottom times.

I hope I've expressed myself coherently.

 

[StSomewhere]

The Suunto RGBM computers are Buhlmann with adjustment factors for RGBM. The adjustment factors are basically there to mimic RGBM in the event of "mistakes" like rapid ascents, reverse profiles, etc. The D9 is the same thing but with deep stop adjustment factors included (plus the "cool factor" gizmos). FWIW, the D9 and the Vytec allow you to lessen the penalizing effect of these RGBM adjustments.

The more interesting question to ask is why do you need RGBM if you aren't doing decompression (i.e. past the NDL) dives?

 

[IanH]

Thank you for the reply

So (in your opionin) does that mean that if you don't make any "mistakes" when using a D9 it will not need to use the RGBM adjustment factors and it will not be so "conservative" ?

Is not a question of needing RGBM, I only dive NDL and inteind to continue that way. I am interested in the subject and wasn't aware that RGBM is not (or less) applicable to non-deco diving.

Ian

 

[Mark Powell]

Is not a question of needing RGBM, I only dive NDL and inteind to continue that way. I am interested in the subject and wasn't aware that RGBM is not (or less) applicable to non-deco diving.

If you only dive NDL then the differences between the various deco models will have very little impact. One may specify a slightly different no-stop time to another but that's about it.

The real difference in deco theories comes in when you start to do deco stops. Very simply a Buhlmann or other Modified Haldane type model will pull you up close to the surface and get you to do extended stops at 9m, 6m and 3m.

A deep stop type model including Gradient Factors, Bubble Models (RGBM/VPM) or even Pyle stops will give you a first stop much deeper.

 

[IanH]

Thanks, Mark

 

[Dr Deco]

Hello readers:

RGBM-like

If you do not perform any deep dives, there are not any residual tissue nuclei about which you need to worry. [These are partially accounted for in the RGBM model.] The advantage of this algorithm is that it will help you in dives below, say, 150 fsw.

It is always important to remember that all diving algorithms put in adjustable parameters to produce a real response to real dive data. Thus, adding adjustable parameters that produce an RGBM-like decompression profile is, in my opinion, just fine.

RGBM

Not wishing to enter into long discussions here, but the parameters of the model are adjusted because the model is less than perfect. I would not go so far to say that it is wrong, jsut somewhat off the mark in places.

However, it is presented to imply that it gives a very accurate biophysical picture of what occurs at the tissue, and lower, level. With this, I would have disagreement on several points.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology is September 10 – 11, 2005 :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm